Pulp-treating process



UNITED STATES PATENT OFFICE eEonqE A, nronrnn, or BERLIN, NEW Hamrsnmn,

ASSIGNOR TO BROWN COMPANY,

OF BERLIN, NEW HAMPSHIRE, A; GORROBATION OF MAINE PULP-TREATING PROCESS1T0 Drawing.

This invention relates to the treatment of a sulphite pulp, which'hasbeen prepared under special conditions, to yield a product havingcertain optimum characteristics to be hereinafter described.

In my application, Serial No. 749,564, filed November 12, 1924,. it isdisclosed that when wood chips (spruce, for example) are cooked in anacid sulphite solution in which theffree S0 and the combined S0 arepresent in substantially equal proportions (or the free S0 less inproportion than combined S0 but are present in sufficient proportion tothe weight of the solution to act uponthe encrustinfiior cementitiouscontent of the wood for a su cient period of time in a closed digester,at a temperature higher than that ordinarily employed. in sulphitedigestion, a pulp is produced which has unusual characteristics andwhich possesses features of advantage over the usual or commercial form"of sulphite pulp. First, the product has longer fibers and a muchgreater strength. than the usual commercial sulphite pulp; and, second,it is comparable in average length of fiber and strength with sulphateor kraft pulp, but has the advantages thereover, that it is not so darkin color and requires less beating to condition it for use in themanufacture of paper. A sheet formed of this pulp, with or withoutbeating, has a shiny or glossy appearance and has a decided rattle whenshaken. Its appearance is markedly different from that of eithercommercial sulphite or sulphate pulp, and as stated, it possesses thebest characteristics of both sulphite and sulphate pulps in theparticulars mentioned. Moreover, the new pulp contains a higherproportion of the wood than is contained in either sulphate orcommercial sulphite pulp. Hence, in addition to cellulose fibers,the-product contains a large proportion of lignin and pentosan, which bytheir presence doubtless impart to the product certain identifyingcharacteristics hereinbefore noted.

To produce pulp such as has beendescrlbed, the wood chips are cooked inan acid suiphite solution having a relatively high comblned SO contentsay, 3% to 4%) with free SO no greater in proportion than the combinedApplication filed December 1, 1925. Serial m, 72,602.

S0 at a temperature of about 320 F. and a total pressure of about to 95pounds. 7 The cooking operation is carried out at this temperature andressure for about ten toeleven hours, tests being made in the usualmanner to determine the progress of the operation and to ascertain whenthe chips have been cooked to the desired extent. In such a process, thecellulose fibers are substantiallyuninjured, since the hydrolysis of thefibers is less than where there is an excess of free S0 and hence theaverage length of fiber is greater than when the acid cooking is carriedon with an excess of, free S0 whereas, on the other hand, the pulpcontains much of the fiber that in an'alkaline digestion would bedissolved in the alkaline liquor. I r

I present herein a table which indicates the characteristics which asulphite pulp produced by the method described possesses, thesecharacteristics beingrepresented by average figures obtained in testingsamples from dif- U ferent cooks:

Uharactem'stics of 4-4 W2 Time of beating -50 minutes Strength (Mullen)170-182 Tear 92-140, Pentosan 5.0% to 6.0% Alpha cellulose 88 7 -89Copper number 2-3 Thus it will be seen that a surprising strength ofpulp is secured, which is comparable to that of kraft or sulphate pulp,which usually possesses a Mullen strength (beaten) of about 155 to 165.

I have further discovered that When'such sulphite pulp, herein termed a4-4 pulp, is treated with a concentrated solution of an alkalinecompound, such as caustic soda, at

about room temperature, the alkaline solution containing sufficientalkali to react with the lignin, less-resistant celluloses, and certaintoo high a concentration of caustic soda must a be avolded, sinceotherwise the alpha cellulose pulp does not tend to hydrate or 1' ell,the beating operation merely tending to macerate or shred the fiber, andresulting in a paper of low strength. On' the other hand, if too' lowsoda solution,

a caustic .concentration is employed, a sufficient purification orreaction with the 1mpurities contained in the pulp, by the caustic willnot take place and the resulting product will-not have undergone asuflicient purification and increase 1n tear test.

In other words, the concentration of caustic soda employed in thealkaline treatmentof the 4-4 pulp is preferably maintained between twolimits, namely, etween a ma x1- mum concentration'at which mercerizat ontakes place, and a minimum concentrat on at which a sufiicient increase'in' tear test of the. pulp is not efiected. These limits will varysomewhat, depending upon the exact temperature or differentcharacteristics of the raw 4-4 stock undergoing the treatment.

Afterthe purification treatment, the pulp is washed free from thealkaline treating agent and may then be bleached to a high'deree ofwhiteness with the usual amount f l ypochlorite bleach, without material1njury to its strength or decrease in cellulose content. If a pulp ofmaximum whiteness is desired, the bleached pulp may finally be. Isuperbleached in a chlorine solution.

\Vhile I have indicated that the purification or refining treatment isordinarily carried out at room temperature, I desire to have it clearlyunderstood that lower than room temperature (say, 0 C. to 25 C.) ma beemployed in the treatment of the-4-4 pulp. Likewise,

higher temperature (for example, 30 C. to A 50 C.) an a correspondinglysuitable concentration of caustic soda, may be em 10 ed in thetreatment. Moreover, an a ka ine liquor containing any other suitablealkaline compound or compounds, such as caustic potash and sodiumsulphide, ma be employed in the purifying treatment, if esired, one or pmore of these compounds being used to relace a portion or all of thecaustic soda. For lnstance, substantially equivalent results to thosepreviously outlined may be obtained by treating a 4-4 stock with asolution of sodium sulphide of somewhat higher concentration than asolution of caustic soda necessary for effecting the same results. Orwhite liquor such as utilized in a kraft mill for the roduction of kraftpulp,-and containing sodium sulphide, sodium hydroxide, and

a slight amount of sodium sulphate and sodium carbonate, may befrequently used with certain advantages.

Where the causticsoda is replaced by a substantial proportion oftreatment employed and the mercerization of the alpha cellulose contentof the pulp undergoing treatment is precluded, since sodium sulphidedoes not cause a mercerization of cellulose fiber. In certain cases itmay be advantageous to incorporate into the cooking liquor, sulphur assuch, or in the form of reducing sulphur compounds, such as sodiumsulphide or sodium thiosulphate, in order to maintain a reducingcondition in the mass during the purifying treatment. Such a condition,it is believed, tends to .improve the yield and the qualities desired inthe final product. At any temperature, however, where a solutionof'eaus'tic soda is em loyed for the purifying treatment, the"max1mumconcentration of caustic soda which is preferably employed is below thatat'which mercerization occurs, andthe minimum above that at-which asuflicient increase in tear test is not efiected. For optimum results inrespect to alpha cellulose content in the finished product and in thetear test of the product, at a treating temperature 175% caustic sodabased on the weight of dry I It may be advantageous to employ arelatively small proportion of oxidizing agent or bleach prior to or inthe alkaline treatment or purification of the H pulp, especially where.the product is to be subsequently bleached. Such treatment renders thepulp more easily .bleachable in the" subsequent bleaching operation, andis especially desirable whe the 4-4; pulp is especially high in ligne Siand other coloring substance. Ordinarily, about 2% to 5% of 35% limebleach (CaOCl based on the dry weight of pulpwhen used in the alkalineliquor is sufiicient to render the pulp easily bleachable in-.subsequent operation. If desired,-however, chlorine maybe introducedinto the alkaline treating solution to form the desired amount of sodiumbleach in solution. Or the pulp may be treated for a relatively shortperiod of time prior to the alkaline treatment, with an oxldizing agent,preferably in the form of a bleach or chlorine solution, depending uponplant expediency, about 2% to 4% of. lime bleach, or 1% to 3% chlorine,being sufficient to render the difiiculty removable nonalpha celluloseconstituents contained in the pulp susceptible to removal in thesubsequent treatment with the alkaline liquor. Or the pul may bepretreated, after which it is prefera I washed, and then treated with analkaline liquor, as reviously described. Of course, where an al alineliquor containing 'a reducing a ent such as'sodium sulphide is employed,leach or other oxidizing agent maynot be added thereto.

In carrying out the treatment in accordlab 16 content, inthe practice ofthe present invenance with this invention, a l4= pulp produced bydigestion in an acid sulph te liquor under the conditions hereinbeforedescribed is Washed substantially free of its digesting liquor and spentproducts of digestion and is preferably thenscreened prior to thealkaline treatment. It is then thickened and intimately admixed with analkaline solution of caustic soda to a consistency of about 5% to 6%,the stock suspension containing about 100% to 175% of caustic soda,based on the weight ofdry fiber. While a lower percentage of causticsoda may be employed in cases Where the resulting pulp need not be ofoptimum tear test and of high alpha cellulosic tion it is preferable toemploy a caustic soda concentration between the limits indicated, sothat a suflicient increase in tear test and puri- .mercerization of thefiber.

fication of the fiber may be obtained without impairing or injuring "itspaper-making. characteristics to any material extent, or in other words,without causing a substantial The alkaline treatment is carried outpreferably while stirringor agitating the pulp so as to promotea'uniform reaction, say, at room temperatiirc' (3.), and thepurification may require thirty minutes to two hours, depending uponthe'characteristics 6f the particular stock undergoing treatment. Thespent liquor contains not only a very high percentage of residualunconsumed causticsoda, but various organic substances removed from thesulphite pulp, as indicated. The large portion of spent liquor isreemployed in subsequent treatment of other pulp, and the chem icals inthe remaining liquormay be recovere portion of such spent The washedpulp is of light color, not much different in appearance from the colorof orstrong rag sheet.

'dinary unbleached sulphite pulp.- I present herewith a table-indicatingcertain characteristics of a pulp which has been treated as hereinbeforeescribed:

.Time of beatin 100 minutes Strength (Mul en); 100-140 Tear 240270Pentosan 1.8-3% Alpha cellulose -l v90-9et% From the foregoing table,which represents the average of a number of tests on samples obtainedfrom different batches of pulp treated with liquors of differentconcentrations of caustic soda, it will be seen that the Mullen strengthof the pul has undergone some reduction, but that t e tear test hasincreased considerably. The treated fiber is no longer harsh and rattly;it resembles the feel of a The purified fiber may now be bleached to r ahi h whiteness without materially affecting its strength or tear, bytreating it at a consistency of about 10% to 16% with about weeniebleaching,

5% to 10% of 35% lime bleach (CaOClf is taken to maintain a temperaturenot ex ceeding 90 F. in the bleaching operation, so as to preserve themaximum strength of fiber. If desired, a certain small roportion ofcanstic soda liquor may be a ded to the bleach liquor and pulp in thebleaching operation, to retard the bleaching reaction and thereby elminate the possibility of lowering the alpha cellulose content orinjuring the fiber strength of the stock, due to .too violent bleaching.The bleaching period ranges from eight to twenty hours, depending uponthe characteristics of the fiber undergoing treatment, and producingstock having a color of 95 to 10 The stock thus bleached may be improvedby a superbleaching operation. In superthe preferably washed andbleached high alpha cellulose stock is treated at a consistency of from4% to 6% with about based on the dry weight of pulp. Precaution 0.2% to0.5% chlorine, based on the weight of dry fiber. N o efl'ort need bemade to control the temperature in superbleaching, which may approximatethe temperature of the wateravailable at the time. If it is desired toincrease the rate of superbleaching, the temperature may be increased toa point not higher than. F. After superbleaching, the stock is treatedwith an anti-chlor, as, for example a bisulphite solution, which servesto react with and to neutralize any residualtraces of chlorine orhydrochloric acid present in the stock. The pulp is then washed freefrom reaction products, and may thenbe sent to a wet machine, from whichit is taken off in sheet form, ready for shipment.

Due to its properties and the purif *ing treatment which it hasreceived, the high alpha cellulose may be employed instead o1 rag fiberin the manufacture of high grade 1 currence, since the tear test and itsaccompanyingquality of folding endurance is of extreme importance inhigh grade papers,

whereas the Mullen or strength test is usually of secondary importance.This feature may be better understood from the fact that it isrelatively easy to increase the Mullen or strength test of a paper sheetby suitable sizing. The sizing operation, however, does not affect thetear test so appreciably, so that it is a distinct advantage to increasethe tear test at the expense of the strength of the pulp,

inasmuch as it is possible to increase the strength in subsequenttreatment.

Moreover, paper made from the unbleached purified pulp as hereinroduced, because of its softness of sheet, is a mirably adapted for manyuses to which a harsh or rattl paper could not be put. For'example, sucpaper may be emplopled for the production of artificial reed fi er suchas employed in the manufacture of bab carriages, furniture,

etc. Such a paper s ould have a high tear test, should be soft and yetfirm, and should have sufficient porosity to permit its readyimpregnation with glue or other chemical reagents.

Before concluding, it may be stated that, while I have specified certainconcentrationsof stock and treating compounds in order to more full andadequately describe my invention, t ese concentrations may vary,'de-

pending upon plant equipment and conditions. That is to say, the plantequipment with which the practlce of the present invention was carriedout, permitted the treatment of b-4 stock at a concentration of about 5%to 6%, most efiiciently .and successfully. On the other hand, a stockconcentration of 10% or greater may be treated-under other plantconditions. In such cases, the percentage of caustic soda or otheralkaline compounds utilized in the purification may be' correspondinglydecreased. In other words, the determining factor in carrying out thetreatment or purifying reaction resides in the concentration of thealkaline compounds in the liquor, even though for convenience thesecompounds may frequently be expressed in terms ofdry fiber weight. Thus,a 5% stock suspension treated with. 100% caustic soda based on dryfiber, represents a 5% caustic soda solution. Of course if sodiumsulphide or some other reagent Whlcll isless drastic or effective in itsreaction than caustic soda is used, the percenta e must be increased 1in order to produce a product substantially equivalent to that obtainedby the use of caustic soda.

I do not herein claim the subject matter claimed in my Patent No.1,635,637, issued mgpulp from the spent iquor o digestion;

and treating such pulp, admixed with a solution of caustic soda to astock consistencyJof about 5% to 6%, at about room temperature,

- for about thirty minutes to two hours, saidrocess which comprisesdigesting solutionv containing 100% to 175% caustic soda based on thewei ht of dry fiber.

2. A rocess whic comprises di esting wood ChlPS in an acid sulphiteliquor aving a combined SO content appreciably above that of a saturatedsolution of calcium bisulphite, and treating the resulting pulp at aboutroom temperature with an alkaline solution containing suflicient alkalito react with and dissolve from such pulp a substantial proportion ofits non-alpha cellulose constituent.

3. A process whichrcomprises di esting wood chips in an acid sulphiteliquor aving a combined S0 content appreclably above that of a saturatedsolution of calcium bisulphite and a freeSO, content approximately equalto'its combinedSO, content, treating the resulting pulp with an oxidant,redigesting the oxidant-treated pulp at about room temperature with analkaline solution containmg sufficient alkali to react with and dissolvefrom such pulp a substantial pro ortion of its non-alpha cellulosecontent, and leaching the alkaline-digested pulp. v

4. A process which comprises pulping wood chips in an acid sulphitecooking liquor in which free S0 and combined S0 are present inapproximately equal proportions of 3% and 4%, and treating the resultingpulp at about room temperature with an alkaline liquor containingsuflicient alkali to react with and dissolve from such pul a substantialproportion of its nonalp a cellulose content.

In testimony whereof I have aflixed my signature. I GEORGE A. RICHTER.

of such alkaline reagent

